Tracking error signals are used in optical storage systems to control the position of a radiation beam with respect to a data track of an optical storage medium. In the case of a magneto-optic (MO) optical storage medium, on which data is generally stored in the form of marks of distinct magnetization, an incident radiation beam reflected from the medium can be used to detect the distinct magnetization and thereby the recorded data. Most commercially-available magneto-optic storage media include a preformed diffracting structure, often referred to as a pregroove, groove or guide track, which provides varying amounts of diffraction of the incident radiation beam depending upon the position of the beam relative to a data track. The pregroove is used to generate a tracking error signal which may be, for example, of the type commonly referred to as a push-pull tracking signal. Push-pull and other exemplary tracking signals are described in pp. 172-181 of A. Marchant, "Optical Recording: A Technical Overview," Addison-Wesley Reading, Massachusetts, which are incorporated by reference herein.
FIG. 1A shows a side-sectional view of a portion of a pregrooved magneto-optic storage medium 10 in accordance with the prior art. The exemplary pregrooves consist of a number of alternating raised regions 12 and lower regions 14. Although not apparent from the view shown, the pregrooves are generally arranged on the surface of the medium 10 in the form of a spiral or concentric circles. The pregrooves thus generally coincide with data tracks containing recorded data, and are used by a tracking servo system to maintain the position of the incident radiation beam on a given data track. It should be noted that in FIG. 1A the height of the raised regions has been exaggerated for clarity. The separation between adjacent raised regions, also referred to as track pitch, may be expressed as a duty cycle. Although the example shown illustrates a medium with a pregroove duty cycle of about 50%, other media could utilize other duty cycles, for example, in a range from about 20% to 80%. The position of the incident beam relative to the pregroove can be determined by, for example, detecting variations in diffraction patterns in a return beam reflected and diffracted from the medium.
FIG. 1A also shows a number of downward arrows 16 and upward arrows 17, which correspond to high or low logic states in recorded binary data and indicate the direction of a magnetization vector in a given region of the medium 10. Depending on the direction of the magnetization vector in the marked and unmarked regions of medium 10, a positive or negative amount of Kerr rotation will be applied to the plane of polarization of the return beam if a linearly-polarized incident beam is applied to the medium. A differential detection system may then be used to reconstruct the recorded data from variations in the Kerr rotation present in the return beam.
FIG. 1B shows an exemplary multi-element focus and tracking detector 18 which may be used in an optical head to detect the return beam reflected and diffracted from the medium 10. The return beam is focused on the detector 18 and the amount of intensity detected in the various detector elements is indicative of the position of the incident radiation beam with respect to the pregroove. The multi-element detector 18 includes four detector elements a, b, c and d, each of which generates an electrical signal indicative of the light intensity incident thereon. A pregroove of the type shown in FIG. 1A may be used to provide a push-pull tracking error signal given by c-d, indicating that the signal from detector element d is subtracted from the signal generated in detector element c. When the incident radiation beam is properly positioned on-track relative to the pregroove, the portion of the return beam intensity detected by detector elements c and d will be the same. If the incident beam goes off-track in a cross-track direction 20, an increased amount of intensity will be detected by either element c or element d. The multi-element detector 18 may also provide a spot-size focus error signal given by (a+b)-(c+d). Further detail regarding the multi-element detector of FIG. 1B may be found in U.S. Pat. No. 5,113,386 entitled "Focus and Tracking Error Detector Apparatus for Optical and Magneto-Optical Information Storage Systems," which is assigned to the assignee of the present invention and incorporated herein by reference. Although the detector 18 provides simple and efficient tracking in a magneto-optic system, it generally requires that the medium include a diffracting structure such as the pregroove shown in FIG. 1A.
Other types of optical media, such as write-once media, can generate tracking error signals without the need for a pregroove or other diffracting structure on the medium. As used herein, write-once media are intended to include mass-produced read-only media such as those commonly used in compact disks (CDs). An exemplary write-once tracking technique utilizes diffraction from preformatted tracking marks, also referred to as servo marks, to generate a sampled phase tracking error signal such as that described in pp. 180-181 of the above-cited A. Marchant reference. However, absent the use of, for example, a multi-spot beam with separate tracking detectors, such preformatted marks have generally not been used to provide a tracking error signal for a magneto-optic medium. The tracking techniques used for magneto-optic media-are thus often incompatible with those used for other types of media, such that optical storage media drives typically cannot handle, for example, both magneto-optic and write-once optical media.
A number of techniques have been developed which use mark edge detection to read data stored on magneto-optic media. See, for example, M. Mansuripur, "Detecting Transition Regions in Magneto-optical Disk Systems," Applied Physics Letters, Vol. 55, No. 8, August 1989; M. Levenson et al., "Edge Detection for Magnetooptical Data Storage," Applied Optics, Vol. 30, No. 2, pp. 232-252, January 1991; M. Yamamoto et al., "Diffraction Pattern from Magnetooptical Edges," Jpn. J. Appl. Phys., Vol. 32, pp. 5206-5209, November 1993; and E. Yamaguchi et al., "Edge Shift Characteristics of a Magnetooptical Edge Detection Signal," Jpn. J. Appl. Phys., Vol. 32, pp. 5349-5353, November 1993; all of which are incorporated by reference herein. However, mark edge detection has generally been used to detect in-track edges suitable for generating a read-out data signal. In addition, these techniques have been used primarily with pregrooved magneto-optic media, and therefore have typically not been applied to generate tracking error signals other than those commonly used with pregrooved media. Systems using these edge detection data read-out techniques therefore generally remain incompatible with certain tracking error signals used with write-once and other types of optical media.
As is apparent from the above, a need exists for improved tracking error signal generation in magneto-optic storage systems such that simple and accurate tracking may be provided on magneto-optic media which do not include a pregroove or a similar preformed diffracting structure.